1. Field of the Invention
The present invention relates to a zoom lens system and more particularly to a standard zoom lens system of a small size suited for a single-lens reflex camera.
2. Related Background Art
Recently various kinds of zoom lens systems have been proposed which have a small size and a high magnification. In the field of the so-called standard zoom lens system having a focal length substantially equal to the length of the diagonal line of the image plane, such zoom lens systems have been proposed as (1) a two-lens groups zoom lens system consisting of a negative lens group and a positive lens group, (2) a four-groups zoom lens system consisting of a positive lens group, a negative lens group, another positive lens group and another negative lens group and (3) another four groups zoom lens system consisting of a positive lens group, a negative lens group, another positive lens group and a further positive lens group.
The two-groups zoom lens system (1) (consisting of a negative lens group and a positive lens group) is small and is composed of a small number of component lenses. Its assembling tolerances are relatively large and it can be manufactured at a low cost. Thus, it has been widely used. Most conventional zoom lens systems each have a zoom ratio of around 2. However, the zoom ratio has recently become higher and higher, and a zoom lens system having a zoom ratio of around 2.5 is proposed, for example, in Japanese Laid-open Patent Application No. 1-239516.
The last lens group of the zoom lens system (2) (consisting of a positive lens group, a negative lens group, another positive lens group and another negative lens group) is a negative lens group. Thus, the zoom lens system (2) provides a telephoto-type power arrangement which is suited for miniaturization. A zoom lens system of this type which has a small size and provides a high zoom ratio is proposed, for example, by Japanese Laid-open Patent Application No. 4-309913.
In the zoom lens system (3) (consisting of a positive lens group, a negative lens group, another positive lens group and a further lens group), the distance between the third lens group of a positive power and the fourth lens group of a positive power can be changed in a floating way during the zooming operation so as to suppress variations of aberrations during the zooming operation. Thus, this system is suited for a high zoom ratio. A zoom lens system of this type which is small and has a high zoom ratio is proposed, for example, by U.S. Pat. No. 5,000,551.
When the zoom ratio exceeds 2.5, however, it is difficult to correct aberrations in a normal two-groups zoom lens system (1) (consisting of a negative lens group and a positive lens group). In order to provide such a high zoom ratio, the two-groups zoom lens system requires a large number of component lenses and/or a large construction. The power of each lens group must be large to miniaturize the two-groups lens system and to give it a high zoom ratio. Thus, assembly tolerances are likely to be strict.
As long as aberration corrections are concerned, the four-groups zoom lens system (2) consisting of a positive zoom lens group, a negative lens group, another positive lens group and another negative lens group is similar to a three-groups zoom lens system consisting of a positive lens group, a negative lens group and another positive lens group. Thus, the four-groups zoom lens group (2) has difficulty in suppressing variations of aberrations, particularly variation of curvature of field.
The four-groups zoom lens system (3) (consisting of a positive lens group, a negative lens group, another positive lens group and a further positive lens group) provides a high zoom ratio and a high efficiency. When, however, there occurs displacement (shift decentering) and/or inclination (tilt decentering) of the optical axes of the third and fourth lens groups, image sharpness is likely to be much deteriorated. In order to avoid such deterioration, the assembly tolerances between the third and fourth lens groups are apt to be strict, making it difficult to reduce the manufacturing cost.